This invention relates to a direction display apparatus which displays the direction of a certain specified location as viewed from a vehicle.
A system of this type has heretofore been, for example, disclosed in Japanese Patent Application Laid-open No. 56-35008. Such a system comprises an azimuth sensor for detecting terrestrial magnetism to measure the heading of the vehicle, a speed sensor for detecting the speed of the vehicle from the revolutions of the vehicle wheels, a calculation portion for executing calculations on the basis of signals from these sensors and signals from a keyboard for inputting data, and a display portion for displaying calculated results, and in which the relative position coordinate data of a starting location and a goal location is input beforehand. As the vehicle travels, its position at any instance from the starting location is found on the basis of the signals of the azimuth sensor and the speed sensor, and the residual running distance of the vehicle to the destination along with the direction of destination as viewed from the vehicle are displayed on the display portion.
In another direction display system, as described in Japanese Patent Application Laid-open No. 56-132513, it is recognized that Islamites make it a religious practice to worship Mecca at predetermined times every day anywhere they are. To fulfill this faithful act, they are required to know the direction of Mecca as viewed from wherever they are. As a result, the system disclosed in Japanese Patent Application No. 56-132513 is capable of determining the direction of one position relative to another. In particular, a magnetic needle turnably supported on a base plate and a map having the curves of equal bearings around Mecca depicted thereon are utilized. As in the foregoing system, when Mecca is set as the destination with its known position coordinate data, the direction of Mecca viewed from any location will be displayed on the display portion at the location.
However, since the computation of the position of the vehicle at any instance using the heading signal and the distance signal is performed by adding up an easterly or westerly movement component and a southerly or northerly movement component based on the heading of every unit distance, the cumulative errors of the added values indicative of the position will increase as the running distance increases. Furthermore, the errors of the heading detection attributed to a disturbance of the magnetic field, the inclinations of a car body, etc. and the errors of the distance detection attributed to dispersion in the outside diameter of wheels, slip, etc., will also affect the determination of the destination direction. Specifically, the direction of Mecca indicated by the display portion may contain a great error. Accordingly, the relative position coordinate data representing the position of the vehicle at an instance as the starting location and the position of Mecca as the destination need to be frequently input anew to avoid cumulative errors or before the errors enlarge. Such data, however, must be obtained by determining relative distances and directions (southerly/northerly or easterly/westerly) between Mecca and the position of the vehicle at different instances. This determination requires the cumbersome tasks of measuring the distances and directions from a map with a rule or special measuring tools and then computing the actual distances and directions using a contraction scale. Moreover, the position coordinate data thus obtained must be input by the use of a keyboard or similar inputting means.
Accordingly, prior-art systems have a problem in that the process for inputting the position coordinate data becomes very complciated, time consuming, and expensive.